Choromytilus chorus and Aulacomya ater are among the most common mussel species in the South Pacific. However, little sclerochronological analysis has been conducted on them, even though they are found in both archaeological sites and sub-fossil deposits and could potentially be useful paleoclimate proxies. One valve from each species was analyzed via x-ray diffraction (XRD) and Raman spectroscopy to determine the shell's mineralogy. C. chorus is mostly calcite with aragonite in some zones, while A. ater is mostly aragonite with less calcite. There are abrupt and continuous boundaries between the two calcite layers and across the aragonite-calcite interface of C. chorus. All of the boundaries in C. chorus can be seen in cross section under reflected or transmitted light along the longest growth axis. The boundaries between mineralogical and structural variations in A. ater are not as visually distinct, and thus may render the species problematic for oxygen isotope analysis. Comparison of C. chorus oxygen isotope profiles to regional temperature records and local water δ18O suggest the sampled shells' lifespan was between one and two years of age. Analyses of the oxygen isotope profiles indicate nearly continuous shell growth throughout the first year of life, with a marked decrease or periods of cessation in shell growth in the second year in the larger specimens analyzed. This growth pattern may make the valves useful as proxies for at least one year of paleoclimatic/paleoenvironmental data. Three out of the four specimens of C. chorus analyzed for oxygen isotopes display a prominent growth break on the outer surface of their valves and each appear roughly contemporaneous. Oxygen isotope analysis indicates that the prominent growth breaks in each of the three valves were formed just before the peak SST of January of 2006 occurred. Qualitative seasonal variations are recorded in the δ18O profiles of the four shells, and median annual δ18O values are in general agreement between individuals. However, the full range of the winter to summer seasonal shift in SST was not recorded by all C. chorus valves. Therefore, caution should be exercised when utilizing C. chorus valves to interpret changes in seasonality.